1. Field of the Invention
This invention relates to the chemical arts. More particularly, this invention relates to a process and system for recovering sulfur from a gas stream rich in hydrogen sulfide.
2. Discussion of the Related Art
It is known in the prior art to recover elemental sulfur from hydrogen sulfide (H2S)—containing acid gas streams as is set forth in the article “Fundamentals of Sulfur Recovery by the Claus Process” by B. Gene Goar, published in the 1977 Gas Conditioning Conference Report. The Claus reaction is represented by the following equation:4H2S+2SO2→3S2+4H2O  (Equation 1)
Claus sulfur recovery units (SRU's) are widely utilized to recover sulfur from acid gas streams produced in natural gas purification and in petroleum refineries, primarily from amine sweetening. In refineries, the H2S is in crude oil and is contained in hydrocarbon desulphurization unit off gases and fluidized catalytic cracker unit off gases. Oftentimes, the acid gas stream produced from the amine unit is quite rich in H2S, particularly in petroleum refineries, where it may be 80-95 mol % H2S. Also in many refineries, the Claus plant units are either fully loaded or subject to becoming fully loaded (capacity limited) due to the processing of heavy crude oils, which contain relatively large amounts of sulfur compounds. With the dwindling known reserves of refinable hydrocarbons and crude oils, less attractive known oil reserves are now being processed, which less attractive oil reserves typically have high sulfur contents. The trend in refining such high sulfur containing feedstocks will increase in the future. Additionally, the requirements to produce lower sulfur fuels will result in more acid gases containing H2S. Therefore, it is a desideratum to increase the capacity of Claus plants to process sulfur.
In conventional Claus sulfur recovery systems, the feed pressure of the acid gas feed stream is only about 12 psig. This low pressure level does not provide enough driving force to allow a significant increase in the amount of acid gas feed that can be passed through the many items of equipment that constitute a typical Claus SRU. As Claus SRU feed rates are increased above capacity, several problems develop. At increased flow, the pressure drop through the Claus plant and tail gas cleanup unit increases, and the back pressure increase requires H2S and air feed inlet pressures beyond what is available from the amine regenerator that supplies the acid gas feed and the air blower that provides feed air. The increased flow also decreases the residence times and increases the space velocity in the reaction furnace and the catalytic reactor stages, which reduces conversion to sulfur and increases emissions to the tail gas cleanup unit. The increased flow also results in overloading some or all of the heat exchangers in the SRU, which may reduce conversion of H2S to sulfur and also increases sulfur vapor carryover to the tail gas unit. The increased flow to the tail gas cleanup unit increases its pressure drop and further lowers tail gas sulfur recovery, which ultimately leads to increased and usually unacceptable sulfur emissions. The increased back pressures may in some Claus plants pose the risk of blowing the liquid sulfur drain seals, which would release process gas containing highly toxic H2S into the atmosphere. While booster blowers for the H2S and air feeds, and higher pressure sulfur liquid drain seals can provide some increase in capacity, these measures will not overcome problems associated with undersized heat exchange equipment, reduced sulfur conversion, or increased sulfur emissions.
It is also known to use oxygen enrichment in the operation of a Claus sulfur plant in order to increase the capacity of H2S handled as well as the total throughput of the plant as set forth in the article “Oxygen Use in Claus Sulfur Plants” by M. R. Gray and W. Y. Svrcek published in the 1981 Gas Conditioning Conference Report. In that article, it was disclosed that oxygen can be added in the air feed to the burner of a Claus reaction furnace in order to increase the amount of H2S which is combusted to sulfur dioxide (SO2) for later catalytic conversion, with additional hydrogen sulfide to the elemental liquid sulfur product of the Claus process. The combustion reaction of H2S with oxygen (whether pure oxygen or air) can be represented by the following equation:2H2S+3O2→2SO2+2H2O  (Equation 2)
The Gray and Svrcek article recites that the pressure drop through the plant and the reactor space velocities determine the maximum capacity increase which can be achieved with oxygen enrichment. Consequently, it is a desideratum to improve efficiency by reducing the amount of air, thereby maximizing the amount of oxygen available to react with the H2S.
However, a further limitation set forth in the Gray and Svrcek article is that for a given plant stream, temperatures and sulfur condenser capacity may limit the potential capacity increase using oxygen enrichment. Specifically, stream temperatures in the reaction furnace and in the converter beds may increase due to oxygen enrichment and, in fact, such increase from oxygen enrichment reaches the maximum tolerable temperature of the materials used in such a furnace, namely the refractory lining. Similarly, in the 1983 publication by Linde of Union Carbide entitled “Claus Plant Oxygen Enrichment,” it is noted that oxygen-enrichment limitations exist for H2S-rich streams due to temperature limits in the furnace or waste heat boiler of a Claus plant. Therefore, it is also a desideratum to moderate the temperature in the Claus reaction furnace.
It is also known in the prior art to recycle effluent gases back into the Claus reaction furnace. For example, U.S. Pat. No. 3,681,024 discloses the addition of oxygen and a recycle gas to a Claus reaction furnace. Combustion gases from a reactor unit are first sent to a water scrubber to reduce the water content of the effluent, and a sufficient amount of the scrubber off-gases are then recycled to dilute the oxygen feed so that furnace conditions are essentially equivalent to operation with air.
U.S. Pat. No. 3,822,341 describes a Claus plant using oxygen enrichment in which water is removed from the combustion gases, first in a liquid vapor contractor and then in an SO2 stripper, before the reaction gases are recycled to a waste heat boiler.
U.S. Pat. No. 4,756,900 discloses a process for splitting the effluent from the waste heat boiler of a Claus reaction furnace and recycling a portion thereof using a separate sulfur condenser and a mechanical blower to moderate the high furnace temperatures induced by oxygen enrichment.
U.S. Pat. No. 4,552,747 describes a process for moderating the high temperatures in a Claus reaction furnace induced by oxygen-enrichment by passing reaction gases through a sulfur condenser and then using a mechanical blower to recycle the resulting effluent stream back to the Claus reaction furnace. U.S. Pat. No. 6,508,998 describes a process for moderating the high temperatures in a Claus reaction furnace induced by oxygen-enrichment by passing reaction gases through a sulfur condenser and then using an eductor to recycle the resulting effluent stream back to the Claus reaction furnace. In some embodiments, steam is used to provide the motive power for the educator.
There remains a definite need for a simple and effective system and process for recovering elemental sulfur from H2S-containing gas streams that minimizes the amount of air required in a Claus reaction furnace. There remains a further definite need for such a system and process that allows the processing of more acid gas through the system. There remains a still further definite need for such a system and process that moderates the temperature in the Claus reaction furnace. The present invention satisfies these and other needs, and provides further related advantages.